A telescopic shaft of a steering mechanism of a vehicle is required to have a property of absorbing an axial displacement which is generated when the vehicle is running and of preventing such displacement or vibration from being propagated onto a steering wheel. Further, the telescopic shaft is also required to have a function of moving the position of the steering wheel in the axial direction and of adjusting this position in order to obtain an optimal position for the driver to drive the vehicle.
In any of these cases, the telescopic shaft is required to reduce rattling sound, to decrease backlash feeling on the steering wheel, and to reduce a sliding resistance during a sliding operation in the axial direction.
For such reasons, conventionally, a male shaft of the telescopic shaft is coated with nylon film and grease is applied onto a sliding portion thereof, so as to absorb or mitigate metallic noise, metallic rattle and, at the same time, to reduce a sliding resistance and backlash in the direction of rotation.
However, there is a case that abrasion of the nylon film advances with use and the backlash in the direction of rotation becomes great. Also, under the high-temperature condition inside the engine room, a nylon film is changed in volume, so that a sliding resistance becomes conspicuously great or the abrasion is notably quickened sometimes, and resultantly, the backlash in the direction or rotation may become great in such a case.
On that account, in Japanese Patent Application Laid-Open No. 2001-50293, between at least one pair of axial grooves respectively formed on the outer peripheral surface of a male shaft and on the inner peripheral surface of a female shaft, there is formed rolling elements (spherical members) which are rotated when both shafts are relatively rotated in the axial direction.
Between the axial groove on the male shaft side and the rolling elements (spherical members), a raceway element which is in contact with the rolling elements (spherical member) and an elastic member for preloading the rolling element (spherical member) against the male shaft and the female shaft through this raceway element are interposed.
With this arrangement, since the rolling element (spherical member) is preloaded against the female shaft by the elastic member to the extent that no backlash is generated when torque is not transmitted (at the time of sliding), it is possible to prevent backlash between the male shaft and the female shaft, whereby the male shaft and the female shaft can slide in the axial direction with a stable sliding load without backlash.
Also, since it is arranged such that the rolling element (spherical member) can be retained in the circumferential direction by the elastic member when torque is transmitted, the male shaft and the female shaft can transmit the torque in a state of high rigidity by preventing backlash in the direction or rotation thereof.
However, in Japanese Patent Application Laid-Open No. 2001-50293 described above, since the rolling element (spherical member) is in point contact with the axial groove of the female shaft and the raceway surface element on the male shaft side, there is sometimes generated a dent (or impression) on the surface of the axial groove, or the like, which is in contact with the spherical member if the contact pressure becomes excessively great when the torque load is great. As a result, an increased fluctuation of the sliding load or deteriorated torsional rigidity is brought about.
With an increase of the torsional torque, a lateral slide of the raceway element with respect to the male shaft may be increased so that hysteresis becomes excessively great.
Further, an amount of preloading and the torsional rigidity may be fluctuated due to a manufacturing error (dimensional error) of each of the male shaft, the female shaft, the elastic member, and the raceway surface element.